Betta Fish: I found the sources for this image, if you show them to neglectful betta owners they will hopefully take it more seriously.

Body text copy pasted from a conversation with Wicked Bettas on insta, shoutout to them for excellent customer service and high standards.

Here are some sources:

Tank size:

https://pmc.ncbi.nlm.nih.gov/articles/PMC10936361/

This source recommends 2.5g as the bare minimum. We recommend at least 5-10 gallons for a few reasons: they're easier to maintain (need less frequent water changes) because waste doesn't build up as quickly. And from our own anecdotal experience, most of our bettas explore their whole tank.

High protein/live feeding and growth:

https://www.researchgate.net/publication/376562513_Effect_of_Different_Natural_Feeding_Blood_Worm_Silk_Worm_and_Water_Fleas_on_The_Growth_of_Betta_Fish_Betta_sp

We tend to recommend frozen/thawed as it is easier to find for most people and they still contain a higher protein content. 

Water temperature:

https://www.seatemperature.org/asia/philippines/malay.htm

They're from South East Asia. This site links to temps in Thailand.

Appendix B: General Fish Aquaculture

Information (page 47, goes into nitrogen cycle/filters, and temperature importance, page 47 in the pdf):

https://www.researchgate.net/publication/280938590_Zebrafish_Danio_rerio_Husbandry_and_Colony_Maintenance_at_the_Northwest_Fisheries_Science_Center#pf25